1. Field of the Invention
The present invention relates to a surface treatment of a metallic product such as a tool and a machine part, and particularly relates to a method for a surface treatment including a single step aimed to harden the surface of a metallic product and to increase fatigue strength thereof by heat treatment and hardening and by generating a residual compressive stress on the surface thereof and to exhibit advantages of enhancing the quality of the surface by heat treatment, of enhancing the residual compressive stress deeper inside the surface thereof and of relieving the surface roughness, without the need to conduct many shot peening steps or a treatment step, such as a polishing step, after a peening step.
2. Description of the Related Art
Conventionally, there has been known, as a method for a surface treatment of a metallic product, shot peening by which all of or part of a metallic product, such as a cast steel product, a casting product and a stainless steel product which are formed into a spring or product shape, is subjected to quench-and-temper treatment and then to cold working. In the shot peening method, a product is quenched at about 850.degree. C. by means of high frequency induction heating and tempered at about 600.degree. C., to thereby transform the surface structure of the product. Thereafter, the resultant product is subjected to air-cooling and to normal peening at ordinary temperature or to warm peening to generate a residual compressive stress, thereby increasing fatigue strength.
In the above-stated shot peening, a plastic deformation resulting from an impact which occurs when injecting a shot on the surface of the metallic product causes a residual compressive stress on the surface of the metallic product. Thus, the residual compressive stress is proportional to the size of a depression which is the plastically deformed portion. The size of the depression or plastically deformed portion is also proportional to the diameter of a shot, so that the residual compressive stress is proportional to the shot diameter, as well.
That is, to provide a residual compressive stress of a portion deep inside the surface layer and to harden the metallic product deeper inside, use of a shot of large particle diameter, conventionally about 1.2 to 0.6 mm, was useful.
In the specification, a shot of a diameter of about 0.3 mm or more is referred to as "large shot" and a shot of a diameter of less than 0.3 to about 0.03 mm is referred to as "small shot".
Further, in the above-stated surface treatment method, it is required to separate a heat treatment step from a shot peening step. Due to this, step management involving temperature control tends to be complicated and cost tends to increase accordingly. To overcome these disadvantages, the present applicant already developed "a surface working and heat treatment method for a metallic product" (Japanese Patent No. 1594395). In this patent, 40 to 200.mu. shots of a hardness equal to or higher than that of a metallic product are injected on the surface of the metallic product at an injection speed of 100 m/sec or higher, the temperature in the vicinity of the surface is increased to be higher than an A.sub.3 transformation point, blasting is conducted to thereby harden the surface of the metallic product following the generation of a residual compressive stress on the surface thereof and to increase the fatigue strength, and heat treatment is conducted to thereby improve the quality of the surface.
The conventional surface treatment method, however, still has the following disadvantages to be further solved.
As stated above, the conventional surface treatment method requires using a relatively large diameter shot so as to obtain a residual compressive stress, for work-hardening or heat treatment hardening the metallic product deep inside the surface of the metallic product. However, if the shot diameter is larger, the shot has disadvantageous in that the service life become shorter and in that the shot cracks more frequently.
The reason is as follows. The momentum of the shot injected at the same speed increases proportionally with the cube of the shot diameter. Therefore, an impulse resulting from the strike is also proportional to the cube of the shot diameter. The area of the destructive part when the shot cracks is proportional to the square of the shot diameter and yield strength is also proportional to the square of the shot diameter. Thus, it is clear that a larger shot tends to crack more easily and that its service life is, therefore, shorter.
If a shot easily cracks, manufacturing cost increases and stable injection is not ensured. Further, the cracking shot causes a failure in a shot peening apparatus. Besides, if the shot is larger, an impact applied on the apparatus itself increases, thereby causing not only shot cracking but also cost increase as a result of damages to the apparatus.
Further, the cracking shot has a sharp corner at the end of the cracking surface. If the cracking shot strikes the surface of a metallic part, it does not produce a depression but enters the surface to cause a cutting action, thereby resulting in the rougher surface of the metallic product.
Conventionally, cast iron shots, cast steel shots and cut wire shots are mainly used. Their service lives are limited.
Moreover, if shot peening is applied on the surface of a metallic product using large shots, the metallic product has a aventurine roughened surface. As a result, if a shot is larger in diameter, the surface become roughened further. Additionally, the large shot tends to crack and the cracking shot cuts the surface of the metallic product to make the surface rougher. With the surface roughened, the metallic product may not be available for use. Also, the residual compressive stress below the surface of the metallic product or product to be treated cannot be obtained.
To solve the above-stated disadvantages, after hard shot peening using large shots is conducted, peening using smaller shots is conducted. Alternatively, after peening, CBN polishing is conducted to relieve the surface roughness and to enhance a residual compressive stress below the surface. In either case, a plurality of treatment steps are required and cost increase is inevitable.
Additionally, according to "surface working and heat treatment method for a metallic product" (Japanese patent No. 1594395), a shot having a diameter of 40 to 200.mu. is utilized to attain high injection speed based on the relationship between the injection speed and injection density. With this method, however, there is a limit to the depth below the surface of the metallic product by which residual compressive stress occurs and the product is hardened by heat treatment.
The present invention has been developed to overcome the above-stated disadvantages. It is, therefore, an object of the present invention to provide a surface treatment method for a metal part which can generate a residual compressive stress in a position below the surface of a metallic product and deeper than the surface layer, which can relieve surface roughness by conducting shot peening having advantages of generating a residual compressive stress to thereby conduct hardening heat treatment to the surface of the metallic product and to enhance fatigue strength thereof by blasting treatment using mixture shots comprised of hard strength, hard hardness material and including small shots and large shots of different shot diameters, and having an advantage of enhancing the quality of the surface by means of heat treatment, and which method can, in particular, dispense with multiple-step shot peening or a treatment step such as polishing after peening as seen in the conventional method.